1. Field of the Invention
The invention relates to a surface-modified solid, to a process for preparing it, and to its use.
2. Background Art
In the surface modification of solids, whether they be pulverulent solids or massive (bulk) solids, a problem which frequently arises is that the modification is not permanent, i.e., the surface modifying agent is not sufficiently anchored onto the surface and is therefore detached and washed off by solvent or a surrounding medium or matrix. This problem can be overcome by firm chemical attachment associated with the formation of chemical bonds. However, this approach has the drawback that, at least as far as known processes are concerned, side reaction products are formed and subsequently eliminated, and disrupt further application. Purification is always associated with increased effort, since it constitutes an additional workstep which increases costs. Moreover, the side reaction products must be disposed of as waste, a costly and environmentally burdensome operation. Added to this is the fact that the purification is usually incomplete, and so unwanted side reaction products still remain on the surface. Additionally, purification, which for technical reasons must take place at a high temperature, imposes a burden on the target product and results in a deterioration in its quality.
Residual amounts of side reaction products are especially deleterious in several application areas. For example, when a pulverulent solid is used as a rheological additive in liquid media such as polymers and resins or resin solutions, as free-flow aids and triboelectric charge control agents in pulverulent systems such as toners, developers, or in pulverulent varnish or coating systems, side reaction products may alter the effects of the additives unpredictably; and when the solid is surface-treated with the aim of improving adhesion or crosslinking with the surrounding medium, side reaction products may alter these properties unpredictably as well.
In U.S. Pat. No. 5,458,916, the addition of alcohols is disclosed as a catalyst for improving the reaction yield of silica with chlorosilanes. This process has the disadvantage that traces of alcohol, as well as the HCl and side reaction products thereof, remain on the metal oxide. In U.S. Pat. No. 3,868,345 and U.S. Pat. No. 3,953,487, metal oxides are silylated with silazanes. With the processes described in these patents, however, traces of ammonia remain on the metal oxide. A disadvantage of these treated metal oxides arises in the course of use as active filler or rheological additive in liquid systems, polymer systems, and resin systems of medium and high polarity. In such systems, problems occur in connection with miscibility and viscosity stability. A further disadvantage is evident when used as additives, e.g., as antiblocking agents, as free-flow aids, or for controlling or regulating triboelectric properties with solids, especially finely divided solids such as crosslinked polymer and resin systems, or finely divided inorganic solids. Problems result with respect to the stability of the system with respect to charge. This causes difficulties when used as free-flow aids and/or charge regulators in pulverulent products such as powder resin systems, powder coatings, or toners and developers.